Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder. Direct fuel injection (DI) systems may improve cylinder charge cooling so that engine cylinders may operate at higher compression ratios without incurring undesirable engine knock. Port fuel injection (PFI) systems may reduce particulate emissions and improve fuel vaporization. In addition, port injection may reduce pumping losses at low loads. To leverage the advantages of both types of fuel injection, engines may also be configured with each of port and direct injection. Therein, based on engine operating conditions, such as engine speed-load ranges, fuel may be delivered via only direct injection, only port injection, or a combination of both types of injection. For example, during an engine restart, the engine may be fueled with each of port and direct injection, with the split ratio adjusted based on one or more engine operating conditions.
One example approach for operating an engine with dual fueling capabilities is shown by Bidner et al. in U.S. Pat. No. 8,100,107. Therein the split ratio for engine fueling includes a higher portion of the fuel mass commanded during an engine cold-start being provided via port injection, and a remaining smaller portion being provided via direct injection. By increasing the ratio of port injected fuel in the fuel split, particulate matter emissions are reduced.
However the inventors herein have identified potential issues with such an approach. As one example, during an engine start, as combustion occurs on a first few events counted since the first combustion event of the engine, engine speed may or may not increase predictably. The speed profile may be affected by numerous factors including engine temperature, component wear causing changes in friction, spark plug degradation, fuel quality, low battery voltage causing slow cranking speeds, etc. Engines may be calibrated to start with larger fuel masses in the first fueling events/engine cycles until the engine exits cranking speeds. If the threshold for exiting cranking engine speed is exceeded in the middle of the fueling cycle for one or more cylinders, and if the desired fuel mass decreases during this fueling cycle, the dual fueled engine may choose to honor the lower desired fuel mass by trimming the fuel pulse commanded to the DI fuel injector. As a result, a target split ratio between the PFI and DI injector is not preserved during this combustion event. In particular, the DI fuel mass may be decreased (or eliminated) if the desired fuel mass decreases by a large amount as the engine exits cranking speeds, or if the decrease is commanded late in the port fueling window (when port injection adjustments are not possible). The deviation from a calibrated split ratio for fuel delivery can have a significant effect on mixture formation. In addition, the deviation from the calibrated split ratio can have cascading effects on other engine operating parameters, such as a deviation from a calibrated spark timing. As a result, combustion stability and robustness may be affected during engine starts. Further, the engine start reliability and repeatability may be reduced.
In one example, some of the above issues may be addressed by a method for an engine comprising: for a first number of consecutive combustion events counted from a first combustion event of an engine start from rest, fueling an engine with each of port and direct injection; and maintaining a ratio of fuel injected via port injection relative to direct injection over the first number of combustion events even as fuel mass changes. In this way, the calibrated split ratio can be prioritized during the engine start until the cranking speed is reached, and then the calibrated fuel mass can be prioritized.
As one example, during an engine start from rest, the engine may be fueled via each of port and direct injection. A calibrated split ratio of fuel delivered via port injection relative to direct injection may be determined based on engine conditions at the engine start (such as engine temperature). For the first combustion event of the start, as well as for a first number of combustion events counted as occurring consecutively after the first combustion event (with no intervening combustion events in between), the calibrated fuel split ratio may be maintained even as fuel mass changes. For example, if a decrease in fuel mass is commanded, the fuel mass is decreased by trimming both the port injection (PFI) fuel pulse and the direct injection (DI) fuel pulse proportionately so that the split ratio is maintained. For example, the end of injection timing of both the PFI and DI fuel pulses may be advanced. As such, this may be possible if the commanded decrease in fuel mass is received earlier in the port injection fueling window (e.g., before an abort angle of the port injection window is reached). If the commanded decrease in fuel mass is received later in the port injection fueling window (e.g., after the abort angle is reached), trimming of the port injection pulse may not be possible. In this case, instead of trimming the DI fuel pulse to provide the commanded fuel mass at the expense of the commanded split ratio, the DI fuel pulse is maintained so as to maintain the commanded split ratio at the expense of the commanded fuel mass. That is, the actual fuel mass delivered may be higher than the commanded fuel mass. After the first number of combustion events have elapsed, the commanded split ratio may be varied to accommodate changes in a commanded fuel mass.
In this way, a more robust engine calibration may be provided across engine starts, even as factors that could affect the start change. By selectively disregarding a commanded decrease in fuel mass received in the middle of a combustion event during engine cranking, a calibrated fuel split ratio may be maintained for a defined number of combustion events counted from the engine start. As such, this reduces variations in mixture formation and deviations from a calibrated spark timing. By prioritizing the commanded split ratio over the commanded fuel mass for the defined number of combustion events from the start, engine start variability arising from sudden changes in fuel mass may be reduced. Overall, engine start combustion stability is improved. In addition, engine starts are made more reliable and repeatable.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.